How to update timeout of a map element?

[Piotr Jurkiewicz <piotr.jerzy.jurkiewicz@xxxxxxxxx>]

Hi,

I have a question regarding a very basic thing: How to refresh the map 
element timeout without recreating that element (i.e. without setting 
its value again)?

According to:

https://wiki.nftables.org/wiki-nftables/index.php/Updating_sets_from_the_packet_path

in the case of sets, the update operation can be used:

update @ctrs { ip saddr . ip daddr . th sport . th dport counter }

However, in the case of maps, the update operation requires specifying 
not only the key, but also value of a map element:

update @flows { ip saddr . ip daddr . th sport . th dport : rt nexthop }

Such a value can be unavailable in many cases. For example, let's 
consider the following configuration which caches flows nexthops (aka 
poor man's flow table):

table ip x {

    map flows {
            typeof ip saddr . ip daddr . th sport . th dport : ip daddr
            timeout 5s
    }

    set ctrs {
            typeof ip saddr . ip daddr . th sport . th dport
            flags dynamic
            timeout 5s
    }

    chain prerouting {
        type filter hook prerouting priority -300; policy accept;
        ip protocol { tcp, udp } dup to ip saddr . ip daddr . th sport 
. th dport map @flows update @ctrs { ip saddr . ip daddr . th sport . th 
dport counter } drop
    }

    chain forward {
        type filter hook forward priority 0; policy accept;

        ip protocol { tcp, udp } add @flows { ip saddr . ip daddr . th 
sport . th dport : rt nexthop }
        ip protocol { tcp, udp } add @ctrs { ip saddr . ip daddr . th 
sport . th dport counter }
    }
}

We can update the @ctrs set in prerouting chain to collect per-flow 
counters. Elements in this set will expire 5s after last flow packet. 
However, there is no way to refresh timeouts of @flows map elements. We 
cannot use rt nexthop here, since it would recreate the entry with 
current nexthop, which would obviously make our cache no op. As a 
result, element in @flows map will expire 5s after FIRST packet and the 
entry would be recreated in forward chain with a new nexthop, which also 
makes the cache pretty useless.

I see two solutions for this problem. The first one would be to allow 
update operations on maps to refresh timeouts without specifying 
element's value:

update @flows { ip saddr . ip daddr . th sport . th dport }

The second one would be to add a new flag to maps, which would make maps 
refresh their elements timeout on every hit (map lookup).

Or there is already some way to achieve this which I missed?

Regards,
Piotr Jurkiewicz